The present invention relates to a new radioactive logging technique for use in a wellbore. More particularly, the invention concerns methods of evaluating a formation when an aqueous solution containing a radioactive tracer is injected into the formation.
At various stages of well completion and for different reasons, solutions may be injected into the formation surrounding the well. For example, treating agents are injected to consolidate loose formations about a perforated portion of the well casing in order to prevent production of sand and the like. In such sand consolidation treatments, and in similar treatments where fluid is injected into a formation, it may be desirable to include a radioactive tracer in the fluid to permit monitoring of the dispersion of the fluid in the formation with a radiation detector.
Fluids containing radioactive tracers are sometimes injected to derive information about the formation. Such injections are not for the purpose of treating the formation, as in a sand consolidation treatment, but rather to permit monitoring by a radiation detector of fluid movement in the formation in order to determine, for example, the location of a fracture or the amount of residual oil in the formation.
The above-described radioactive tracing procedures are complicated due to the fact that many formations include naturally-occurring radioactive elements. Thorium-232, uranium-238, uranium-235, their daughter nuclides, and potassium-40 constitute the great majority of radioactive elements which occur naturally. When a radiation detector is used to detect movement of a tracer-bearing fluid which has been injected in to a formation, gamma-ray emissions from naturally occurring elements in the formation may be detected by the radiation detector.
Most of the naturally-occurring radioactive elements emit gamma rays at several different energies. Sodium iodide crystals have been used in the past as well logging detectors. Such past detectors are designed to detect gamma ray energies over a selected energy spectrum. A tracer nuclide is selected that has at least one gamma-ray emission energy which is in the spectrum to be detected. The selected spectrum is broad enough so that gamma-ray emissions from naturally present elements are also detected. In order to obtain data relating to the dispersal of the tracer, a base log must be run prior to injection of the tracer. After the tracer is injected, a second log is run and the two logs are subtracted to delete information relating to gamma-ray emissions which are not produced by the tracer. Such past methods tend to produce somewhat inaccurate results because of the inherently poor resolution of this type of detector, i.e., approximately .+-.10%.
It is an object of the present invention to provide a method for monitoring a radioactive tracer in a formation with increased accuracy and selectivity.
It is another object of the invention to provide such a method wherein a base log need not be run.
The present method includes the steps of injecting a fluid containing a radioactive tracer into a formation. The selected tracer emits at least one gamma-ray having an energy different from any of the gamma-ray energies emitted by naturally occurring radioactive elements which may be present. After fluid injection, a logging sonde with a high resolution detector is used to detect tracer radiation in the formation. Such detectors have been used in the mining industry to measure the uranium ore content of a formation surrounding a test borehole. The selected energy detection spectrum of these detectors includes at least one gamma-ray emission energy of the tracer and excludes substantially all gamma-ray emission energies of the naturally occurring radioactive elements.
These and other objects and attendant advantages of the present invention will become apparent as further consideration is given to the following detailed description and accompanying drawings.